1. Field of the Invention
The present invention relates generally telecommunications transmissions systems and more particularly to a peak detector for T1 span equipment.
2. Description of Related Art
Many telecommunication transmission systems include a central office (CO) that may transmit useful data, or xe2x80x9cpayload,xe2x80x9d signals over transmission lines to equipment on customer premises. Typically, digital payload signals are sent over the transmission lines through an office repeater a series of regenerative repeaters, to a network interface unit (NIU), and in turn to customer premises equipment (CPE). Similarly, payload is carried from the CPE to the NIU and in turn to the CO.
The NIU typically sits at the point of demarcation between the telephone operating company""s side of the telephone line and the customer""s side of the telephone line. In general, the NIU is electrically transparent to payload signals. However, the NIU can be used for special maintenance functions such as loopback and performance monitoring.
The Bell telephone system in the United States, for instance, has widely utilized a digital time-domain multiplexing pulse code modulation system known as the T1 transmission system. Each T1 transmission system carries 24 8-KB/second voice or data channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction. T1 transmission systems are used in multiples xe2x80x9cNxe2x80x9d, thus providing Nxc3x9724 channels on Nxc3x972 cable pairs.
FIG. 1 illustrates in general the arrangement of a telecommunications transmission system 100 including an NIU. As shown in FIG. 1, system 100 includes a central office 120, an NIU 140 and a CPE 160. A first pair of tip and ring cables 180, 220 carries signals from the central office to the customer premises and is referred to as the receive or xe2x80x9cRCVxe2x80x9d line. A second pair of tip and ring cables 240, 260 carries signals from the customer premises to the central office and is referred to as the transmit or xe2x80x9cXMTxe2x80x9d line.
Thus, in regular operation, an NIU may receive signals from the network via a xe2x80x9cRCV INxe2x80x9d line 280 and may then pass those signals via a xe2x80x9cRCV OUTxe2x80x9d line to the CPE 300. Similarly, the NIU may receive signals from the CPE via an xe2x80x9cXMT INxe2x80x9d line 320 and may then pass those signals via an xe2x80x9cXMT OUTxe2x80x9d 340 line to the central office. Of course, these designations are made only for convenience and may change depending on the perspective of an observer.
In the T1 system, the data to be transmitted over the lines, such as speech, is sampled at a rate of 8,000 hertz, and the amplitude of each sample is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into binary form. Representative binary pulses appear on the transmission lines. The binary form of each sample pulse consists of a combination of seven pulses, or bits. An eighth bit is periodically added to allow for signaling.
A coding system is typically used to convert the analog signal to a digital signal. The system guarantees some desired properties of the signal, regardless of the pattern to be transmitted. The most prevalent code in the United States is bipolar coding with an all zero limitation (also called Alternative Mark Inversion or xe2x80x9cAMIxe2x80x9d). With bipolar coding, alternating ones (high bits) are transmitted as alternating positive and negative pulses, so as to assure a direct current balance and avoid base line wander. Further, an average density of one pulse in eight slots, with a maximum of fifteen zeros between xe2x80x9cones,xe2x80x9d is required. This is readily obtained in voice-band coding, however, by simply not utilizing an all zero word. Contrasted with bipolar coding is unipolar coding, in which every occurrence of a high bit is seen as a positive pulse.
In many telecommunication systems, data may be transmitted sequentially in discrete groups of bits called xe2x80x9cframes.xe2x80x9d In the T1 system, for instance, each of the 24 channels in the T1 system is sampled within a 125 microsecond period (equivalent to {fraction (1/8000)}) of a second, constituting one frame. A synchronizing bit, or xe2x80x9cframe bit,xe2x80x9d is added to each frame to serve as a flag, enabling line elements to distinguish each frame from the preceding frame or from noise on the line. Since there are 8 bits per channel and there are 24 channels and one frame bit at the end of each frame, the total number of xe2x80x9cbitsxe2x80x9d needed per frame is 193. Thus, the resulting line bit rate for T1 systems is 1.544 million bits per second.
Signals that violate either the coding rules or the framing rules established in a particular system are detected as errors. Thus, for example, under a bipolar coding scheme, two positive pulses should never occur in sequence. To the extent such pulses do occur adjacent to each other, such a signal may be noted as a bipolar coding violation. Similarly, a digital signal that violates framing rules (such as framing bit requirements) established in a given system is detected as a xe2x80x9cframe error.xe2x80x9d In a given encoding protocol, a sufficient number of frame errors may be detected as a frame loss.
In telecommunications transmission systems, it is occasionally necessary to monitor the performance characteristics of a particular transmission line. By monitoring transmission line performance, service providers can proactively respond to facility performance degradation and can therefore improve the telecommunications circuit.
Being positioned at the point of demarcation between the network and CPE, an NIU can be conveniently configured to assist with performance monitoring in several ways. For example, an NIU can be arranged to provide a xe2x80x9cmaintenance loopbackxe2x80x9d function, in which the NIU shunts back incoming signals to the direction from which they came. Loopback can be used to test the continuity and performance of the transmission system up to and including the NIU, either from the network (CO) side or the customer (CPE) side, by allowing a remotely positioned test set to determine whether a signal sent to the NIU returns unaltered.
To establish loopback on the network side, for instance, the central office may send a xe2x80x9cloopupxe2x80x9d command to an NIU, instructing the NIU to enter loopback. In that event, the NIU would internally switch the signals that it receives from the network (on the RCV IN line) onto the line that transmits to the network (the XMT OUT line), so as to shunt signals from the central office back to the central office. In this state, if the same test signal that is sent down the transmission line from the central office to the NIU for a substantial period of time is received back by central office, then the central office can be substantially assured that the two way transmission system between the central office and the NIU is functioning properly. Alternatively, if the same signal applied to the transmit line does not return along the receive line, then the central office can determine that an error or malfunction has occurred at a point along that T1 line.
When an NIU enters network loopback, the transmission of signals along the receive line from the central office to the CPE is interrupted. Consequently, during that period, an NIU will typically generate and apply to the RCV OUT line an alarm indication signal (AIS) (e.g., a continuous sequence of all 1""s), indicating a loss of the network signal.
In addition to performing loopback, an NIU can serve other performance monitoring functions as well. For example, an NIU can monitor the signals being transmitted between the central office and CPE in order to identify the presence of transmission errors such as bipolar violations or out of frame conditions. The NIU may maintain a record of these errors for later reporting to a service technician or test system.
As another example, an NIU may be arranged to monitor the signals being transmitted along the transmission line in order to identify and respond to predefined control codes. These control codes may represent requests for information or may comprise instructions to take various actions. In response to such codes, the NIU may return transmission performance statistics, report its operating state, change its operating state or take other action.
As still another example, an NIU may be arranged to monitor the signals being transmitted along the transmission line in order to determine when a loss of signal (LOS) occurs on either the network or CPE side. Typically, in response to a LOS on one side, an NIU will generate and apply an AIS to its outgoing line on the other side, so as to alert the other side that a LOS exists. For instance, if an NIU stops receiving signals on its XMT IN line, the NIU may automatically transmit an AIS via its XMT OUT line, to notify the central office that signal transmission from the CPE has stopped.
In addition, an NIU can be arranged to regenerate and build out the signals that it receives before forwarding the signals on to another location. For this purpose, an NIU typically includes a regenerator and a line build out (LBO) circuit. The regenerator may operate to amplify an input signal to account for attenuation along the transmission line and to substantially recreate the original signal. The LBO, in turn, may simulate the attenuation and wave shaping that a signal would normally endure as it passes through cable, to reduce cross talk among bundled lines. Thus, for instance, when the NIU receives a signal from the central office, the NIU may regenerate the T1 signal and then apply it to the LBO circuit (which simulates the attenuation and wave shaping of the T1 signal as it passes through cable) before transmitting the signal to the CPE.
In operation, an NIU should be able to measure the peak level of an incoming AC signal, so as to be able to reconstruct an output wave representing the signal, or add a fixed amount of LBO loss relative to the input signal level, and thereby simulate a passive LBO function.
A peak detector typically comprises a comparator having as inputs an AC signal and a DC reference. The comparator output is established based on a measure of the difference between these two input signals. For instance, in one arrangement, whenever the AC signal is more positive than the DC reference, the comparator may provide a positive output, and whenever the AC signal is less than or equal to the DC reference, the comparator may provide no output. Various other arrangements are possible as well.
The peak detector circuit produces a DC output substantially representative of the peak of the incoming AC signal. To do so, the output of the comparator is typically used to charge an output capacitor to a representative DC output level, and the DC output is in turn used as a basis to establish the DC reference input to the comparator. Thus, as the AC signal level varies in relation to the DC reference, the comparator output varies and may therefore change the level of both the DC output and the DC reference input. By designing the capacitor circuit with appropriate charging and discharging time constants, the peak detector can be made more or less sensitive to variations in the AC input.
Limitations exist, however, in using the output of a comparator, alone, to charge the output capacitor. For instance, the output transistor stage of a typical comparator will act as a current sink rather than a current source. Consequently, the comparator output would only charge an output capacitor negatively and provide a negative DC output voltage relative to the negative peaks of the input waveform. Where the output of the peak detector is to be provided in turn to an analog-to-digital converter or other device that expects a positive input, such negative peak detector output is undesirable.
Instead of using only the comparator output to charge the capacitor, some existing peak detectors are arranged to charge the output capacitor via either an NPN transistor or a JFET controlled by the output of the comparator. In these arrangements, the comparator output turns the transistor on and off and thereby selectively turns on or off a current supply to the capacitor. In operation, the charging transistor is typically off most of the time, once the peak level has been established. Only when the detected signal exceeds the feedback reference will the transistor conduct, thereby causing the capacitor to charge to a higher DC voltage. The NPN transistor is kept in the non-conducting state by the application of a negative voltage on the base terminal relative to the transistor emitter. This is in turn accomplished by the comparator being turned on and having a low voltage output. The low output causes a current to flow through biasing resistors, and the corresponding voltage drop across the biasing resistors provides the negative base voltage to keep the charging transistor in an off state.
Thus, in a peak detector that uses an NPN transistor as the charging element, the peak detector uses considerable current and power due to the constant current used to establish the biasing voltage of the charging transistor. Furthermore, the NPN transistor depends on a resistor pull-up to provide current to the transistor base. Unfortunately, however, this arrangement is slow to turn on and provides little base current and consequently little current drive to charge the output capacitor. That is, after the comparator turns off, the NPN transistor turns on only after the base voltage has been pulled high by the supply voltage through the biasing pull up resistor. To increase the turn on speed, the biasing resistor should be low valued. However, the low resistive value means an even higher biasing current flow when it is desired to keep the transistor in the off state. Typically, the design trade off is selected to achieve power efficiency over speed. Therefore, this arrangement is not very responsive to high speed signal inputs.
Alternatively, the peak detector may use an n-channel JFET as the charging element. FIG. 2 illustrates a prior art peak detector using a JFET as the charging element. As shown in FIG. 2, a comparator is tied to a negative voltage, and the output of the comparator feeds the gate of an n-channel JFET, which in turn feeds a DC output capacitor. Like the NPN transistor, the JFET is kept in the off condition (for the majority of the time) by the application of a negative voltage. Thus the comparator is conducting most of the time to draw a bias current through a pull-up resistor R3, resulting in a negative voltage being applied to the gate. In addition, the JFET gate capacitance varies considerably, and the pinch-off voltage threshold of the JFET also vary considerably, which leads to variations in response time of the peak detector to higher frequency input waveforms.
The JFET configuration has at least three disadvantages: (i) it requires a large voltage swing (from xe2x88x925V to +10V) to turn on and begin charging the capacitor, (ii) the circuit as a whole has parasitic capacitive effects that impede this voltage change and hence cause an even slower response, and (iii) the on and off thresholds of the JFET are not consistent from part to part. While the JFET does not require base current, it does require a pull-up resistor R3 to bring the base voltage high, and current will flow through resistor R3 as long as the JFET is held off (which is most of the time). The voltage change from xe2x88x925V to +10V is a significant voltage variation to achieve in a high-speed circuit.
The circuit capacitive effects compound the problem. Specifically, the JFET gate capacitance must be discharged. In addition, because the comparator output is typically the collector of a transistor within the comparator, the capacitance of this transistor must also be discharged. That is, when the comparator is outputting a low voltage to keep the JFET off, the transistor in the comparator is on and the output of xe2x88x925V appears on the transistor collector. To then transition the JFET to the conductive state, the collector has to move from xe2x88x925V to +10 V. Therefore, to turn on the JFET quickly, resistor R3 must be relatively small and therefore drawing more current when the JFET is off (which is most of the time).